1. Field of the Invention
This invention relates to a control system for synchronous drive of a DC motor, and more specifically to a control system for forced synchronization in which whenever the speed of the DC motor varies from a predetermined synchronous speed governed by a reference signal, the speed may be brought back quickly to the synchronous speed as desired.
2. Description of the Prior Art
DC motors are divided into so-called DC commutator motors using a commutator and brushes for distributing and supplying electric power from a DC source to the field windings and so-called brushless motors, without using such brushes, having a rotor with magnetic poles opposite in polarity, a field system with coils for producing magnetic flux, and a flux detector such as a Hall effect element and the like for switching over power from the DC source and supplying it to individual coils.
There have been presented various methods of controlling speed of the DC commutator motor, which are put to practical use. In most of such conventional methods motor speed is detected, and output thus obtained is compared with a reference speed for producing a signal corresponding to any difference therebetween so that a speed control circuit may be operated with the signal. This is not suitable for high precision speed control because of high drift.
On the other hand, some synchronous drive systems for prior-art DC brushless motors have been devised, which cannot, however, eliminate the phenomena of so-called "step out" effectively. Such phenomena may often occur in the following way:
1. As a DC motor of this type approaches its synchronous running state, it tends to be synchronized with some of the higher or lower harmonics of a reference signal frequency for synchronous operation, and consequently synchronous speed cannot be obtained as desired.
2. Even if the average power supplied to the driving coil of the DC motor is as much as that for normal synchronous operation, pulse power may not be such as to have equal pulse width, and thus so-called "beat signals" varying periodically tend to often occur. Under this condition the motor may be kept anyhow at a constant speed, which is different from a predetermined speed. This speed may vary directly when the load is changed. Thus, no constant speed can be obtained.